1/1×4+1/4×7+1/7×10+···+1/17×20
=1/3x(1-1/4+1/4-1/7+1/7-1/10+...+1/17-1/20)
=1/3x(1-1/20)
=19/60
1/1×4+1/4×7+1/7×10+···+1/17×20
=(1/1-1/4+1/4-1/7+1/7-.................-1/20)/3
=(1-1/20)/3
=19/60
1/1×4+1/4×7+1/7×10+···+1/17×20
=1/3×(1-1/4+1/4-1/7+1/17-1/20)
=1/3×(1-1/20)
=1/3×19/20
=19/60
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